The LDL receptor-related protein (LRP) is an extremely large (600,000 kDa) single chain protein with four related extracellular domains that are in turn composed of multiple (2, 8, 10, 11 respectively) cysteine-rich ligand binding repeats. LRP has emerged as a unique endocytic receptor due to its ability to bind and endocytose a number of structurally and functionally distinct ligands. These include apolipoprotein E, 2-macroglobulin, tPA, and urokinase-plasminogen activator. A 39kDa receptor-associated protein (RAP) co-purifies with intracellular LRP and blocks ligand binding to LRP in vitro. Preliminary data provided in this proposal demonstrate that the different ligands bind to multiple and distinct regions on LRP and LRP binding sites on RAP. In addition, many cells that synthesize LRP and RAP also synthesize ligands for the receptor, which can prevent the correct folding of LRP if they associate to early. The P.I. hypothesizes that RAP serves a chaperoning function in the folding and proper trafficking of LRP by binding it and preventing premature association with ligand. Experiments are proposed to elucidate the molecular mechanism by which RAP interacts with LRP, explore the structural features of RAP by solving its crystal structure, determine the role of RAP as a folding chaperone using in vivo and in vitro methods, and define the trafficking of RAP and LRP both as individual molecules and interacting complexes.